1. Field of the Invention
This invention relates generally to a lumped element balun for a ring mixer and, more particularly, to a circuit layout for a lumped element ring balun, where the balun circuit elements are configured on a monolithic substrate in a manner that conserves space and minimizes parasitic inductances.
2. Discussion of the Related Art
Modern communications systems employ transmitter and receiver designs that attempt to maximize the utilization of the assigned frequency bandwidth associated with the various communications channels because signal bandwidth is a costly investment for the system provider. Maximizing the utilization of the assigned bandwidth translates to providing transmitters and receivers that have extremely high performance. However, the transmitters and receivers must also be low cost. The radio frequency (RF) components in a communications system typically are the highest cost items because they are usually custom designed elements and are not mass produced.
One RF component that falls into this category is a frequency mixer. A frequency mixer mixes two RF or intermediated frequency (IF) signals to create a sum and difference frequency for frequency down-conversion or frequency up-conversion purposes. For example, the signal received in the receiver is mixed with a local oscillator (LO) signal to generate an IF signal suitable for subsequent signal processing. Typically, mixers are critical for setting the performance of the RF signal chain. Thus, mixers with lower intermodulation products and high dynamic range that can be implemented as a cell in an RF integrated circuit (IC) are needed.
One known mixer employed in RF communications systems of the type being discussed herein is referred to in the art as a ring mixer. A ring mixer employs four diodes connected in a ring configuration that mix the RF signal and the LO signal to generate the IF signal. The ring mixer employs a hybrid or balun that splits the RF signal and the LO signal into signals that are 180xc2x0 out of phase with each other. A ring mixer balun is disclosed in Sturdivant, Rick, xe2x80x9cBalun Designs for Wireless, . . . Mixers, Amplifiers and Antennas,xe2x80x9d Applied Microwave, Summer 1993, pps. 34-44. The split RF signals and the LO signals are applied to the mixer between the diodes at opposite corners of the ring. The diodes are switched on and off in response to the positive and negative portions of the RF signals to provide modulation. The IF signal is generated between the diodes at the other opposite corners of the ring.
FIG. 1 is a schematic diagram of a known lumped element ring balun circuit 10. The ring balun circuit 10 includes an electrical ring 12 having four sides defining corner nodes 14, 16, 18 and 20. The ring balun circuit 10 includes an electrical configuration of capacitors C1-C6, inductors L1-L4 and a resistor R1. Each side of the ring 12 includes a capacitor and an inductor that combine to provide a high pass filter that forms a lumped element transmission line that causes a delay of a signal propagating therethrough. As is known in the art, current leads voltage on a capacitor, and voltage leads current on an inductor. Therefore, a series capacitor and shunt inductor provide a phase lead of the signal, and a series inductor and a shunt capacitor provide a phase lag of the signal.
An RF input signal is applied to the node 14, and the filters provide an RF signal at the node 20 that is 90xc2x0 out of phase with the signal at the node 14, an RF signal at the node 18 that is 180xc2x0 out of phase with the signal at the node 14, and an RF signal at the node 16 that is 270xc2x0 out of phase with the signal at the node 14. Output lines 54 and 56 are coupled to the nodes 20 and 16, respectively, to provide output signals that are 180xc2x0 out of phase with each other. DC blocking capacitors 24 and 26 are provided in the output lines 54 and 56 to prevent DC signals from the mixer from entering the ring balun circuit 10.
The ring balun circuit 10 is applicable for a ring mixer, but is limited in use for other types of mixers, such as star mixers and double doubly balanced mixers, because of the complexities in providing a dual balun in the ring design. Therefore, other balun designs are employed in the art for other types of mixers. FIG. 2 is a schematic diagram of a lumped element dual-balun circuit 30 including a first balun 32 and a second balun 34 that has particular application for use in combination with a star mixer or a monolithic microwave integrated circuit (MMIC) double doubly balanced mixer (DDBM). The dual-balun circuit 30 receives an RF input signal, and the first balun 32 outputs two signals that are 180xc2x0 out of phase with each other and the second balun 34 outputs two RF signals that are 180xc2x0 out of phase with each other. A dual-balun structure of this type is disclosed in Chiou, Hwann-Keo, et al., xe2x80x9cMiniature MMIC Star Double Balanced Mixer Using Lumped Dual Balun,xe2x80x9d Electronics Letters, Vol. 33, No. 6, Mar. 13, 1997, pps. 503-505, and Chiou, Hwann-Keo, et al., xe2x80x9cA Miniature MMIC Double Doubly Balanced Mixer Using Lumped Dual Balun for High Dynamic Receiver Application,xe2x80x9d IEEE, Microwave and Guided Wave Letters, Vol. 7, No. 8, August 1997, pps. 227-229.
The dual-balun circuit 30 employs inductor and capacitor filter networks in the same manner as the balun circuit 10 discussed above to provide the RF signals that are 180xc2x0 out of phase with each other. The balun 32 includes a filter made up of inductor L1 and capacitor C1 and a filter made up of inductor L2 and capacitor C2. Likewise, the balun 34 includes a filter made up of inductor L3 and capacitor C3 and a filter made up of inductor L4 and capacitor C4. In the balun 32, the inductor L1 is coupled to the capacitor C1 at node 36, the inductor L1 is coupled to the capacitor C2 at node 38, the capacitor C2 is coupled to the inductor L2 at node 40, and the inductor L2 is coupled to the capacitor C1 at node 42. In the balun 34, the inductor L3 is coupled to the capacitor C3 at node 44, the inductor L3 is coupled to the capacitor C4 at node 46, the capacitor C4 is coupled to the inductor L4 at node 48, and the inductor L4 is coupled to the inductor C3 at node 50. The RF input signal is applied to the nodes 36 and 44. An RF output signal that is in phase with the RF input signal is provided at the nodes 42 and 50, and an RF output signal that is 180xc2x0 out of phase with the RF input signal is provided at the nodes 38 and 46.
In accordance with the teachings of the present invention, a circuit layout for a lumped element ring balun is disclosed where the elements of the balun are patterned on a monolithic substrate in a compact design. The balun includes four inductors and a plurality of capacitors electrically coupled together to filter and delay an RF input signal to provide corresponding RF output signals that are out of phase with each other. The inductors are symmetrically disposed in a rectangular area on the substrate. A first pair of the inductors is positioned at one end of the rectangular area where the inductors are adjacent to each other. A second pair of the inductors is positioned at an opposite end of the rectangular area, where the inductors are adjacent to each other. All of the capacitors are formed on the substrate in a central circuit area between the first pair of inductors and the second pair of inductors.
The design employs metallized traces patterned on the substrate to provide electrical coupling between the inductors and the capacitors, where traces are provided at different levels in the substrate. Each inductor includes a winding having an inner end and an outer end that are electrically coupled to circuit elements in the circuit area. The inner end of each winding is coupled to a trace that extends under the winding through an air bridge to be electrically isolated therefrom. The length of the electrical connections to the capacitors are minimized to reduce parasitic inductances. Ground vias extend through the substrate and are electrically coupled to a metallized ground plane at an opposite side of the substrate.
Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.